Display device and electronic device including the display device

ABSTRACT

To provide a display device including a transistor that includes an oxide semiconductor and has favorable characteristics, a pixel electrode electrically connected to the transistor, and a capacitor electrically connected to the pixel electrode. To provide a display device that can be manufactured at low cost. The display device includes a display element including a pixel electrode, a transistor that performs switching of the display element and includes a first oxide semiconductor layer serving as a channel formation region, a capacitor that is electrically connected to the display element and includes a dielectric layer between a pair of electrodes. The pixel electrode is a second oxide semiconductor layer formed on the same surface as that on which the first oxide semiconductor layer is formed, and also serves as one electrode of the capacitor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a display device and an electronic device using the display device.

2. Description of the Related Art

Transistors used for most flat panel displays typified by a liquid crystal display device and a light-emitting display device are formed using silicon semiconductors such as amorphous silicon, single crystal silicon, and polycrystalline silicon provided over glass substrates. Further, transistors using such silicon semiconductors are used in integrated circuits (ICs) and the like.

In recent years, attention has been drawn to a technique in which, instead of a silicon semiconductor, a metal oxide exhibiting semiconductor characteristics is used in transistors. Note that in this specification, a metal oxide exhibiting semiconductor characteristics is referred to as an oxide semiconductor.

For example, such a technique is disclosed in which a transistor is fabricated using zinc oxide or an In—Ga—Zn-based oxide as an oxide semiconductor and the transistor is used as a switching element or the like in a pixel of a display device (see Patent Documents 1 and 2).

A display device that includes a capacitor in which an oxide semiconductor film provided on the same surface as that on which an oxide semiconductor film of a transistor is provided and a pixel electrode electrically connected to the transistor are provided to be a predetermined distance apart from each other in order to increase the aperture ratio is disclosed (see Patent Document 3).

A method for manufacturing a thin film transistor in which including an oxide semiconductor film provided on the same surface as that on which an oxide semiconductor film serving as a pixel electrode is provided is disclosed (see Patent Document 4). In the manufacturing method, an oxide semiconductor film included in a pixel electrode is not patterned alone; an interlayer insulating film covers an oxide semiconductor film that is included in the thin film transistor, and plasma treatment is performed on an exposed portion of the oxide semiconductor film that is not covered by the interlayer insulating film to reduce the resistance of the oxide semiconductor film that is included in the pixel electrode.

REFERENCE

-   [Patent Document 1] Japanese Published Patent Application No.     2007-123861 -   [Patent Document 2] Japanese Published Patent Application No.     2007-096055 -   [Patent Document 3] U.S. Pat. No. 8,102,476 -   [Patent Document 4] PCT International Publication No. WO2011/010415

SUMMARY OF THE INVENTION

In the display device disclosed in Patent Document 3, a pixel electrode serving as one electrode of a capacitor needs to be formed in a step different from a step of forming an oxide semiconductor layer included in a transistor, which increases the manufacturing cost.

In the method for manufacturing a thin film transistor disclosed in Patent Document 4, there is no technical description of a capacitor, one of components of a display device.

In view of the above problems, an object of one embodiment of the present invention is to provide a novel display device including a transistor that includes an oxide semiconductor and thus has favorable characteristics, a pixel electrode electrically connected to the transistor, and a capacitor electrically connected to the pixel electrode. Another object of one embodiment of the present invention is to provide a display device that can be manufactured at low cost.

Note that the descriptions of these objects do not disturb the existence of other objects. Note that one embodiment of the present invention does not necessarily achieve all the objects. Objects other than the above objects will be apparent from and can be derived from the descriptions of the specification, the drawings, the claims, and the like.

One embodiment of the present invention is a display device that includes a display element including a pixel electrode, a transistor that performs switching of the display element and includes a first oxide semiconductor layer serving as a channel formation region, a capacitor that is electrically connected to the display element and includes a dielectric layer between a pair of electrodes. The pixel electrode is formed using a second oxide semiconductor layer formed on the same surface as that on which the first oxide semiconductor layer is formed, and also serves as one electrode of the capacitor.

Note that the second oxide semiconductor layer serving as the pixel electrode and the one electrode of the capacitor can also be referred to as an oxide semiconductor layer having high conductivity. The oxide semiconductor layer having high conductivity can be formed as follows. When hydrogen is added to an oxide semiconductor including oxygen vacancies, hydrogen enters oxygen vacant sites and forms a donor level in the vicinity of the conduction band. As a result, the conductivity of the oxide semiconductor layer is increased, so that the oxide semiconductor layer becomes conductive. Such an oxide semiconductor layer having become conductive can be referred to as an oxide conductor layer. An oxide semiconductor layer generally has a visible light transmitting property because of its large energy gap. An oxide conductor layer is an oxide semiconductor having a donor level in the vicinity of the conduction band. Therefore, the influence of absorption due to the donor level is small, and an oxide conductor layer has a visible light transmitting property comparable to that of an oxide semiconductor layer.

According to one embodiment of the present invention, the first oxide semiconductor layer serving as the channel formation region of the transistor and the second oxide semiconductor layer serving as the pixel electrode can be formed at the same time. Furthermore, the second oxide semiconductor layer also serves as the one electrode of the capacitor; thus, a step of further forming a conductive layer is unnecessary, reducing the manufacturing steps of the display device. This enables the display device to be manufactured at low cost.

Another embodiment of the present invention is a display device that includes a display element including a pixel electrode, a transistor that performs switching of the display element, a capacitor that is electrically connected to the display element and includes a dielectric layer between a pair of electrodes. The transistor includes a gate electrode, a gate insulating layer over the gate electrode, a first oxide semiconductor layer over the gate insulating layer, and source and drain electrodes over the first oxide semiconductor layer. The pixel electrode is formed using a second oxide semiconductor layer formed on the same surface as that on which the first oxide semiconductor layer is formed, and also serves as one electrode of the capacitor.

In the above structure, the dielectric layer of the capacitor is preferably formed using an insulating layer formed on the same surface as that on which the gate insulating layer is formed. Furthermore, in the above structure, the other electrode of the capacitor is preferably formed using a conductive layer formed on the same surface as that on which the gate electrode is formed.

According to one embodiment of the present invention, the one electrode of the capacitor is formed at the same time as the gate electrode of the transistor. The other electrode of the capacitor is formed at the same time as the oxide semiconductor layer of the transistor. Note that the other electrode of the capacitor also serves as the pixel electrode. In addition, the dielectric layer of the capacitor is formed at the same time as the gate insulating layer of the transistor. Thus, a step of further forming a conductive layer and/or a dielectric layer to form a capacitor and/or a pixel electrode is unnecessary, reducing the manufacturing steps of the display device. This enables the display device to be manufactured at low cost.

According to one embodiment of the present invention, it is possible to provide a novel display device including a transistor that includes an oxide semiconductor and thus has favorable characteristics and, a pixel electrode electrically connected to the transistor, and a capacitor electrically connected to the pixel electrode. Furthermore, the display device can be manufactured at low cost.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a top view illustrating one embodiment of a display device;

FIG. 2 is a cross-sectional view illustrating one embodiment of a display device;

FIGS. 3A to 3C are cross-sectional views illustrating one embodiment of a method for manufacturing a display device;

FIGS. 4A to 4C are cross-sectional views illustrating one embodiment of a method for manufacturing a display device;

FIG. 5 is a cross-sectional views illustrating one embodiment of a display device;

FIGS. 6A to 6C are cross-sectional views illustrating one embodiment of a method for manufacturing a display device;

FIGS. 7A and 7B are a cross-sectional view and a band diagram illustrating one embodiment of a display device;

FIG. 8 is a cross-sectional view illustrating one embodiment of a display device;

FIGS. 9A to 9C are a block diagram and circuit diagrams illustrating embodiments of display devices; and

FIGS. 10A to 10H illustrate examples of electronic devices.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described below in detail with reference to the drawings. Note that the present invention is not limited to the description below, and it is easily understood by those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the present invention. Accordingly, the present invention should not be interpreted as being limited to the content of the embodiments below. In addition, in the following embodiments, the same portions or portions having similar functions are denoted by the same reference numerals or the same hatch patterns in different drawings, and descriptions thereof are not repeated.

Note that in each drawing described in this specification, the size, the film thickness, or the region of each component may be exaggerated for clarity. Therefore, embodiments of the present invention are not limited to such a scale.

In this specification and the like, ordinal numbers such as “first” and “second” are used in order to avoid confusion among components, and the terms do not limit the components numerically. Therefore, for example, the term “first” can be replaced with the term “second”, “third”, or the like as appropriate.

Note that functions of a “source” and a “drain” of a transistor are sometimes replaced with each other when a transistor of opposite polarity is used or when the direction of current flow is changed in circuit operation, for example. Therefore, the terms “source” and “drain” can be used to denote the drain and the source, respectively, in this specification.

Embodiment 1

In this embodiment, a display device of one embodiment and a manufacturing method of the display device of one embodiment of the present invention will be described with reference to FIG. 1, FIG. 2, FIGS. 3A to 3C, FIGS. 4A to 4C, FIG. 5, FIGS. 6A to 6C, FIGS. 7A and 7B, and FIG. 8.

<Structural Example of Display Device>

FIGS. 1 and 2 illustrate a structural example of the display device. FIG. 1 is a plan view of a pixel 301 included in a display device, and FIG. 2 is a cross-sectional view along dashed-dotted lines A1-A2 and B1-B2 in FIG. 1. Note that in FIG. 1, some components (e.g., insulating layers 305 and 306) of the pixel 301, a display element provided over the pixel 301, and the like are not illustrated for simplicity.

In the pixel 301 in FIG. 1, a conductive layer 304 a serving as a scan line extends substantially perpendicularly to a signal line (in the horizontal direction in the drawing). The conductive layer 310 a serving as the signal line extends substantially perpendicularly to the scan line (in the vertical direction in the drawing). A conductive layer 304 b serving as a capacitor line extends in parallel to the signal line.

Furthermore, the pixel 301 in FIG. 1 includes the transistor 102. The transistor 102 is provided at a region where the scan line and the signal line intersect each other. The transistor 102 includes a conductive layer 304 a serving as a gate electrode, a gate insulating layer (not illustrated in FIG. 1), an oxide semiconductor layer 308 a that is over the gate insulating layer and in which a channel region is formed, and conductive layers 310 a and 310 b serving as source and drain electrodes.

The conductive layer 304 a serves as a scan line and a gate electrode; the conductive layer 304 a that overlaps with the oxide semiconductor layer 308 a serves as the gate electrode of the transistor 102. The conductive layer 310 a serves as a signal line and a source electrode; the conductive layer 310 a that overlaps with the oxide semiconductor layer 308 a serves as the source electrode of the transistor 102. In the top view of FIG. 1, an edge of the scan line is on the outer side than an edge of the oxide semiconductor layer 308 a. Thus, the scan line also serves as a light-blocking layer for blocking light from a light source such as a backlight. For this reason, the oxide semiconductor layer 308 a included in the transistor 102 is not irradiated with light, so that a change in the electrical characteristics of the transistor 102 can be suppressed.

Part of the conductive layer 310 b that serves as the drain electrode is over the oxide semiconductor layer 308 b formed at the same time as the oxide semiconductor layer 308 a. Part of the oxide semiconductor layer 308 b overlaps with the conductive layer 304 b that serves as a capacitor line and is formed at the same time as the conductive layer 304 a.

The oxide semiconductor layer 308 b serves as a pixel electrode in the pixel 301.

The pixel 301 illustrated in FIG. 1 includes a capacitor 105. The capacitor 105 includes a dielectric layer between a pair of electrodes. One of the pair of electrodes is formed using the conductive layer 304 b serving as the capacitor line, and the other electrode is formed using the oxide semiconductor layer 308 b. As the dielectric layer of the capacitor 105, an insulating layer formed in the same step as the gate insulating layer of the transistor 102. In other words, the dielectric layer of the capacitor 105 is an insulating layer formed on the same surface as that on which the gate insulating layer of the transistor 102 is formed.

Thus, the oxide semiconductor layer 308 b is formed on the same surface as that on which the oxide semiconductor layer 308 a serving as a channel formation region of the transistor 102, and serves as the pixel electrode and the one electrode of the capacitor 105.

Thus, a step of further forming a conductive layer is not needed to form the capacitor and the pixel electrode, whereby the manufacturing steps of the display device can be reduced.

Next, FIG. 2 is a cross-sectional view along dashed-dotted lines A1-A2 and B1-B2 in FIG. 1.

The display device of one embodiment of the present invention illustrated in FIG. 2 includes a liquid crystal element 322 between a pair of substrates (a substrate 302 and a substrate 342).

The liquid crystal element 322 includes the oxide semiconductor layer 308 b, layers each having an alignment property (hereinafter referred to as alignment films 318 and 352), a liquid crystal layer 320, and a conductive layer 350. The oxide semiconductor layer 308 b serves as one electrode of the liquid crystal element 322, and the conductive layer 350 serves as the other electrode of the liquid crystal element 322. Although a structure where the liquid crystal element 322 includes the alignment films 318 and 352 is described in this embodiment, the liquid crystal element 322 is not limited thereto. For example, a structure where the liquid crystal element 322 does not include the alignment films 318 and 352 may be employed.

Note that in this embodiment, a liquid crystal display device will be described in which a vertical electric field mode is used as a driving method of the liquid crystal element 322.

A “liquid crystal display device” refers to a display device including a liquid crystal element. The liquid crystal display device includes a driver circuit for driving a plurality of pixels, for example. The liquid crystal display device also includes a control circuit, a power supply circuit, a signal generation circuit, a back light module, and the like provided on another substrate, and can be referred to as a liquid crystal module.

The transistor 102 included in the display device in FIG. 2 includes the conductive layer 304 a that serves as the gate electrode, the insulating layers 305 and 306 that are over the conductive layer 304 a and serve as gate insulating layers, the oxide semiconductor layer 308 a over the gate insulating layers, and the conductive layers 310 a and 310 b that are over the oxide semiconductor layer 308 a and serve as the source and drain electrodes. The insulating layers 312 and 314 are provided as protective films over the transistor 102, more in details, over the oxide semiconductor layer 308 a and the conductive layers 310 a and 310 b.

The conductive layer 310 b serving as the drain electrode of the transistor 102 is over and in contact with part of a top surface of the oxide semiconductor layer 308 b. The oxide semiconductor layer 308 b is formed in the same step as the oxide semiconductor layer 308 a so as to be over the insulating layer 306 serving as the gate insulating layer. Note that the oxide semiconductor layer serves as the pixel electrode in the pixel 301.

The capacitor 105 included in the display device in FIG. 2 has a dielectric layer between the pair of electrodes. One of the pair of electrodes is formed using the conductive layer 304 b, and the other electrode is formed using the oxide semiconductor layer 308 b. The dielectric layer of the capacitor 105 is formed using the insulating layers 305 and 306 serving as the gate insulating layers of the transistor 102. In other words, the insulating layers 305 and 306 are provided for both the transistor 102 and the capacitor 105. Portions of the insulating layers 305 and 306 that overlap with the conductive layer 304 a serve as the gate insulating layers of the transistor 102, and portions of the insulating layers 305 and 306 that overlap with the conductive layer 304 b serves as the dielectric layer of the capacitor 105. Although FIG. 2 illustrates a layered structure of the insulating layers 305 and 306, this layered structure is not necessarily employed and a single-layer structure or a layered structure including three or more layers may be employed.

The oxide semiconductor layer 308 a and the oxide semiconductor layer 308 b are layers processed into island-like shapes through the same film formation step and the same etching step. An oxide semiconductor is a semiconductor material whose resistivity can be controlled by oxygen vacancies in the film of the semiconductor material and/or the concentration of impurities such as hydrogen or water in the film of the semiconductor material. Thus, treatment to be performed on the oxide semiconductor layers 308 a and 308 b is selected from the following to control the resistivity of each of the oxide semiconductor layers 308 a and 308 b formed through the same steps: treatment for increasing oxygen vacancies and/or impurity concentration and treatment for reducing oxygen vacancies and/or impurity concentration.

Specifically, plasma treatment is performed on the oxide semiconductor layer 308 b serving as the pixel electrode and the electrode of the capacitor to increase oxygen vacancies and/or impurities such as hydrogen or water in the oxide semiconductor layer 308 b, so that the carrier density can be increased and the resistivity can be reduced. Furthermore, an insulating layer containing hydrogen is formed in contact with the oxide semiconductor layer 308 b to diffuse hydrogen from the insulating layer containing hydrogen to the oxide semiconductor layer 308 b, so that the oxide semiconductor layer 308 b can have a higher carrier density and a lower resistance.

On the other hand, the insulating layer 312 is provided to prevent the oxide semiconductor layer 308 a included in the transistor 102 from being subjected to the plasma treatment. Since the insulating layer 312 is provided, the oxide semiconductor layer 308 a is not in contact with the insulating layer 314 containing hydrogen. The insulating layer 312 can be formed as an insulating layer capable of releasing oxygen, in which case oxygen can be supplied to the oxide semiconductor layer 308 a. The oxide semiconductor layer 308 a to which oxygen is supplied is an oxide semiconductor in which oxygen vacancies in the film or at the interface are filled with and which has higher resistance. Note that as the insulating layer capable of releasing oxygen, a silicon oxide film or a silicon oxynitride film can be used, for example.

As the plasma treatment to be performed on the oxide semiconductor layer 308 b, plasma treatment using a gas containing one of a rare gas (He, Ne, Ar, Kr, or Xe), hydrogen, and nitrogen is typical. Specifically, plasma treatment in an Ar atmosphere, plasma treatment in a mixed gas atmosphere of Ar and hydrogen, plasma treatment in an ammonia atmosphere, plasma treatment in a mixed gas atmosphere of Ar and ammonia, plasma treatment in a nitrogen atmosphere, or the like can be employed.

By the plasma treatment, an oxygen vacancy is formed in a lattice from which oxygen is released (or in a portion from which oxygen is released) in the oxide semiconductor layer 308 b. The oxygen vacancy might cause carrier generation. Further, when hydrogen is supplied from an insulating film that is in the vicinity of the oxide semiconductor layer 308 b, specifically, that is in contact with the lower surface or the upper surface of the oxide semiconductor layer 308 b, and hydrogen is combined with the oxygen vacancy, an electron serving as a carrier might be generated. Accordingly, the oxide semiconductor layer 308 b whose oxygen vacancies are increased by the plasma treatment has higher carrier density than the oxide semiconductor layer 308 a.

The oxide semiconductor layer 308 a in which oxygen vacancies are filled with oxygen and the hydrogen concentration is reduced can be referred to as a highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor layer. The term “substantially intrinsic” refers to the state where an oxide semiconductor has a carrier density lower than 1×10¹⁷/cm³, preferably lower than 1×10¹⁵/cm³, more preferably lower than 1×10¹³/cm³. A highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor has few carrier generation sources, and thus can have a low carrier density. The highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor layer 308 a has a low density of defect states and accordingly can have a low density of trap states.

Furthermore, the highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor layer 308 a has an extremely low off-state current; even when an element has a channel width of 1×10⁶ μm and a channel length L of 10 μm, the off-state current can be less than or equal to the measurement limit of a semiconductor parameter analyzer, i.e., less than or equal to 1×10⁻¹³ A, at a voltage (drain voltage) between a source electrode and a drain electrode in the range from 1 V to 10 V. Thus, the transistor 102 whose channel region is formed in the oxide semiconductor layer 308 a has a small change in electrical characteristics and is highly reliable.

The insulating layer 312 in FIG. 2 is formed in such a manner that the insulating layer that overlaps with the oxide semiconductor layer 308 b serving as the pixel electrode and the electrode of the capacitor is selectively removed. The insulating layer 314 may be formed in such a manner that the insulating layer is formed in contact with the oxide semiconductor layer 308 b and then partly removed. For example, an insulating layer containing hydrogen, that is, an insulating layer that can release hydrogen, typically, a silicon nitride film containing hydrogen, is used as the insulating layer 314, whereby hydrogen can be supplied to the oxide semiconductor layer 308 b. The insulating layer that can release hydrogen preferably has a hydrogen concentration of 1×10²² atoms/cm³ or higher. Such an insulating layer is formed in contact with the oxide semiconductor layer 308 b, whereby hydrogen can be effectively contained in the oxide semiconductor layer 308 b. In this manner, the above-described plasma treatment is performed and the structure of the insulating layer in contact with the oxide semiconductor layer is changed, whereby the resistivity of the oxide semiconductor layer can be appropriately adjusted.

Hydrogen contained in the oxide semiconductor layer 308 b reacts with oxygen bonded to a metal atom to be water, and in addition, an oxygen vacancy is formed in a lattice from which oxygen is released (or a portion from which oxygen is released). Entry of hydrogen into the oxygen vacancy generates an electron serving as a carrier in some cases. Furthermore, in some cases, bonding of part of hydrogen to oxygen bonded to a metal atom causes generation of an electron serving as a carrier. Thus, the oxide semiconductor layer 308 b containing hydrogen has a higher carrier density than the oxide semiconductor layer 308 a.

Hydrogen in the oxide semiconductor layer 308 a of the transistor 102 in which a channel is formed is preferably reduced as much as possible. Specifically, in the oxide semiconductor 308 a, the hydrogen concentration measured by secondary ion mass spectrometry (SIMS) is set to lower than or equal to 2×10²⁰ atoms/cm³, preferably lower than or equal to 5×10¹⁹ atoms/cm³, more preferably lower than or equal to 1×10¹⁹ atoms/cm³, more preferably lower than or equal to 5×10¹⁸ atoms/cm³, more preferably lower than or equal to 1×10¹⁸ atoms/cm³, more preferably lower than or equal to 5×10¹⁷ atoms/cm³, more preferably lower than or equal to 1×10¹⁶ atoms/cm³.

On the other hand, the oxide semiconductor layer 308 a serving as the pixel electrode and the electrode of the capacitor is a low-resistance oxide semiconductor layer that has high hydrogen concentration and/or a large amount of oxygen vacancies as compared to the oxide semiconductor layer 308 b

Note that other components of the display device illustrated in FIGS. 1 and 2 will be described in detail in Method for manufacturing display device.

<Method for Manufacturing Display Device>

Examples of a method for manufacturing the display device illustrated in FIGS. 1 and 2 will be described with reference to FIGS. 3A to 3C, FIGS. 4A to 4C, FIG. 5, and FIGS. 6A to 6C.

First, the conductive layers 304 a and 304 b are formed over the substrate 302, and the insulating layers 305 and 306 are formed over the conductive layers 304 a and 304 b. After that, an oxide semiconductor layer 307 is formed over the insulating layer 306 (see FIG. 3A).

There is no particular limitation on a material and the like of the substrate 302 as long as the material has heat resistance high enough to withstand at least heat treatment performed later. For example, a glass substrate, a ceramic substrate, a quartz substrate, or a sapphire substrate may be used as the substrate 302. Alternatively, a single crystal semiconductor substrate or a polycrystalline semiconductor substrate made of silicon, silicon carbide, or the like, a compound semiconductor substrate made of silicon germanium or the like, an SOI substrate, or the like may be used as the substrate 302. Still alternatively, any of these substrates provided with a semiconductor element may be used as the substrate 302. In the case where a glass substrate is used as the substrate 302, a glass substrate having any of the following sizes can be used: the 6th generation (1500 mm×1850 mm), the 7th generation (1870 mm×2200 mm), the 8th generation (2200 mm×2400 mm), the 9th generation (2400 mm×2800 mm), and the 10th generation (2950 mm×3400 mm); thus, a large-sized display device can be manufactured. Still alternatively, a flexible substrate may be used as the substrate 302, a display element, the transistor 102, and the like may be directly provided on the flexible substrate.

The conductive layers 304 a and 304 b can be formed using a metal material such as molybdenum, titanium, tantalum, tungsten, aluminum, copper, chromium, neodymium, scandium, or the like, or an alloy material that contains any of these materials as its main component. Alternatively, a semiconductor film typified by a polycrystalline silicon film doped with an impurity element such as phosphorus, or a silicide film such as a nickel silicide film may be used for the conductive layers 304 a and 304 b. The conductive layers 304 a and 304 b can each have either a single-layer structure or a layered structure. The conductive layers 304 a and 304 b may each have a tapered shape with a taper angle of greater than or equal to 15° and less than or equal to 70°, for example. Here, the taper angle refers to an angle formed between a side surface of a layer having a tapered shape and a bottom surface of the layer.

The insulating layers 305 and 306 correspond to the gate insulating layer of the transistor 102 and the dielectric layer of the capacitor 105. As each of the insulating layers 305 and 306, an insulating layer including at least one of the following films formed by a plasma CVD method, a sputtering method, or the like can be used: a silicon oxide film, a silicon oxynitride film, a silicon nitride oxide film, a silicon nitride film, an aluminum oxide film, a hafnium oxide film, an yttrium oxide film, a zirconium oxide film, a gallium oxide film, a tantalum oxide film, a magnesium oxide film, a lanthanum oxide film, a cerium oxide film, and a neodymium oxide film. Note that the layered structure of the insulating layers 305 and 306 is not necessarily employed, and an insulating layer with a single-layer structure including any of the above films may be used.

Note that the insulating layer 306 in contact with the oxide semiconductor layer 307 is preferably an oxide insulating layer and preferably has a region (oxygen-excess region) containing oxygen in excess of that in the stoichiometric composition. To provide the oxygen-excess region in the insulating layer 306, the insulating layer 306 can be formed in an oxygen atmosphere, for example. Alternatively, oxygen may be introduced into the formed insulating layer 306 to form the oxygen-excess region therein. As a method for introducing oxygen, an ion implantation method, an ion doping method, a plasma immersion ion implantation method, plasma treatment, or the like may be employed.

In this embodiment, a silicon nitride layer is formed as the insulating layer 305, and a silicon oxide layer is formed as the insulating layer 306. The silicon nitride layer has a higher dielectric constant than the silicon oxide layer and needs to have a larger film thickness than the silicon oxide layer to obtain capacitance equivalent to that of the silicon oxide layer. Thus, when the silicon nitride layer is included in the insulating layer 305 serving as the gate insulating layer of the transistor 102 and the dielectric layer of the capacitor 105, the physical thickness of the insulating layer can be increased. This makes it possible to reduce a decrease in withstand voltage of the transistor 102 and the capacitor 105 and furthermore to increase the withstand voltage, thereby reducing electrostatic discharge damage to the transistor 102 and the capacitor 105.

The oxide semiconductor layer 307 preferably includes a film represented by an In—M—Zn oxide that contains at least indium (In), zinc (Zn), and M (M is a metal such as Al, Ti, Ga, Y, Zr, La, Ce, Nd or Hf). Alternatively, both In and Zn are preferably contained. To reduce variations in electrical characteristics of the transistors including the oxide semiconductor, the oxide semiconductor preferably contains a stabilizer in addition to In and Zn.

Examples of the stabilizer include gallium (Ga), tin (Sn), hafnium (Hf), aluminum (Al), and zirconium (Zr). Other examples of the stabilizer include lanthanoid such as lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu).

As the oxide semiconductor included in the oxide semiconductor layer 307, for example, any of the following can be used: an In—Ga—Zn-based oxide, an In—Al—Zn-based oxide, an In—Sn—Zn-based oxide, an In—Hf—Zn-based oxide, an In—La—Zn-based oxide, an In—Ce—Zn-based oxide, an In—Pr—Zn-based oxide, an In—Nd—Zn-based oxide, an In—Sm—Zn-based oxide, an In—Eu—Zn-based oxide, an In—Gd—Zn-based oxide, an In—Tb—Zn-based oxide, an In—Dy—Zn-based oxide, an In—Ho—Zn-based oxide, an In—Er—Zn-based oxide, an In—Tm—Zn-based oxide, an In—Yb—Zn-based oxide, an In—Lu—Zn-based oxide, an In—Sn—Ga—Zn-based oxide, an In—Hf—Ga—Zn-based oxide, an In—Al—Ga—Zn-based oxide, an In—Sn—Al—Zn-based oxide, an In—Sn—Hf—Zn-based oxide, and an In—Hf—Al—Zn-based oxide.

Note that an In—Ga—Zn-based oxide means an oxide containing In, Ga, and Zn as its main components and there is no particular limitation on the ratio of In, Ga, and Zn. The In—Ga—Zn-based oxide may contain a metal element other than the In, Ga, and Zn.

As a method for forming the oxide semiconductor layer 307, a sputtering method, a molecular beam epitaxy (MBE) method, a CVD method, a pulse laser deposition method, an atomic layer deposition (ALD) method, or the like can be used as appropriate.

In forming the oxide semiconductor layer 307, the concentration of hydrogen to be contained is preferably as low as possible. For example, in the case of a sputtering method, a deposition chamber needs to be highly evacuated and also a sputtering gas needs to be highly purified, in order to reduce the hydrogen concentration. As an oxygen gas or an argon gas used as the sputtering gas, a gas that is highly purified to have a dew point of −40° C. or lower, preferably −80° C. or lower, more preferably −100° C. or lower, still more preferably −120° C. or lower is used, so that entry of moisture or the like into the oxide semiconductor layer 307 can be prevented as much as possible.

In order to remove moisture remaining in the deposition chamber, an entrapment vacuum pump, such as a cryopump, an ion pump, or a titanium sublimation pump, is preferably used. Alternatively, a turbo molecular pump provided with a cold trap may be used. A cryopump has a high capability in removing a hydrogen molecule, a compound containing a hydrogen atom, such as water (H₂O) (preferably, also a compound containing a carbon atom), and the like; thus, the impurity concentration in the film formed in the deposition chamber evacuated with the cryopump can be reduced.

In the case where the oxide semiconductor layer 307 is formed by a sputtering method, the relative density (fill rate) of a metal oxide target that is used for the deposition is greater than or equal to 90% and less than or equal to 100%, preferably greater than or equal to 95% and less than or equal to 100%. With the use of the metal oxide target having high relative density, a dense film can be formed.

Note that formation of the oxide semiconductor layer 307 while the substrate 302 is maintained at a high temperature is also effective in reducing the impurity concentration in the oxide semiconductor layer 307. The heating temperature of the substrate 302 is higher than or equal to 150° C. and lower than or equal to 450° C., and preferably, the substrate temperature is higher than or equal to 200° C. and lower than or equal to 350° C.

Next, the oxide semiconductor layer 307 is processed into a desired shape, so that the island-shaped oxide semiconductor layers 308 a and 308 c are formed (see FIG. 3B).

The oxide semiconductor layer 308 c, which becomes the oxide semiconductor layer 308 b later, and the oxide semiconductor layer 308 a are formed by processing the oxide semiconductor layer 307; therefore, they contain at least the same metal elements. When the oxide semiconductor layer 307 is processed by etching, part of the insulating layer 306 (a region not covered with the oxide semiconductor layer 308 a and the oxide semiconductor layer 308 c) might be etched to be thinned because of overetching of the oxide semiconductor layer 307.

After the island-shaped oxide semiconductor layer 308 a and 308 c are formed, heat treatment is performed. The heat treatment is preferably performed at a temperature of higher than or equal to 250° C. and lower than or equal to 650° C., preferably higher than or equal to 300° C. and lower than or equal to 400° C., more preferably higher than or equal to 320° C. and lower than or equal to 370° C., in an inert gas atmosphere, an atmosphere containing an oxidizing gas at 10 ppm or more, or a reduced pressure atmosphere. Alternatively, the heat treatment may be performed in such a manner that heat treatment is performed in an inert gas atmosphere, and then another heat treatment is performed in an atmosphere containing an oxidizing gas at 10 ppm or more in order to compensate released oxygen. By the heat treatment, an impurity such as hydrogen or water can be removed from at least one of the insulating layers 305 and 306 and the oxide semiconductor layers 308 a and 308 c. Note that the heat treatment may be performed before the oxide semiconductor film 307 is processed into an island shape.

In order to obtain stable electrical characteristics of the transistor 102 including an oxide semiconductor serving as a channel formation region, it is effective to reduce the impurity concentration in the oxide semiconductor so that the oxide semiconductor becomes intrinsic or substantially intrinsic.

Next, the conductive layers 310 a and 310 b and the insulating layer 312 are formed over the insulating layer 306 and the oxide semiconductor layers 308 a and 308 c (see FIG. 3C).

The conductive layers 310 a and 310 b can each be formed to have a single-layer structure or a layered structure using any of single metals such as aluminum, titanium, chromium, nickel, copper, yttrium, zirconium, molybdenum, silver, tantalum, and tungsten, or an alloy containing any of these single metals as its main component. For example, a two-layer structure in which a titanium film is stacked over an aluminum film, a two-layer structure in which a titanium film is stacked over a tungsten film, a two-layer structure in which a copper film is stacked over a copper-magnesium-aluminum alloy film, a three-layer structure in which a titanium film or a titanium nitride film, an aluminum film or a copper film, and a titanium film or a titanium nitride film are stacked in this order, a three-layer structure in which a molybdenum film or a molybdenum nitride film, an aluminum film or a copper film, and a molybdenum film or a molybdenum nitride film are stacked in this order, and the like can be given. Note that a transparent conductive material containing indium oxide, tin oxide, or zinc oxide may be used. The conductive layers 310 a and 310 b can be formed by a sputtering method, for example.

For the insulating film layer 312, a silicon oxide film, a silicon oxynitride film, an aluminum oxide film, or the like having a thickness in the range from 150 nm to 400 nm can be used, for example. In this embodiment, a 300-nm-thick silicon oxynitride film is used as the insulating layer 312. The insulating layer 312 can be formed by a PE-CVD method, for example.

Then, the insulating layer 312 is processed into a desired shape so that an opening 362 is formed (see FIG. 4A).

The opening 362 is formed so as to expose the oxide semiconductor layer 308 c. An example of a formation method of the opening 362 includes, but not limited to, a dry etching method. Alternatively, a wet etching method or a combination of dry etching and wet etching can be employed for formation of the opening 362. The etching step for forming the opening 362 reduces the thickness of the oxide semiconductor layer 308 c in some cases.

After that, heat treatment is preferably performed. By the heat treatment, part of oxygen contained in the oxide insulating layer 312 can be transferred to the oxide semiconductor layer 308 a, so that oxygen vacancies in the oxide semiconductor layer 308 a can be filled. Consequently, oxygen vacancies in the oxide semiconductor layer 308 a can be reduced, while oxygen vacancies in the oxide semiconductor layer 308 c that is not in contact with the insulating layer 312 are not reduced. Thus, the amount of oxygen vacancies in the oxide semiconductor layer 308 c is larger than that of oxygen vacancies in the oxide semiconductor layer 308 a. The heat treatment can be performed under conditions similar to those for the heat treatment performed after the formation of the oxide semiconductor layers 308 a and 308 c.

Next, the insulating layer 314 is formed over the insulating layer 312 and the oxide semiconductor layer 308 c so as to cover the opening 362. Forming the insulating layer 314 changes the oxide semiconductor layer 308 c into the oxide semiconductor layer 308 b (see FIG. 4B).

The insulating layer 314 contains hydrogen. When hydrogen in the insulating layer 314 is diffused to the oxide semiconductor layer 308 c, hydrogen is bonded to oxygen vacancies in the oxide semiconductor layer 308 c and electrons serving as carriers are generated. As a result, the oxide semiconductor layer 308 c has a reduced resistivity to form the oxide semiconductor layer 308 b.

The resistivity of the oxide semiconductor layer 308 b is lower than at least the resistivity of the oxide semiconductor layer 308 a and is preferably higher than or equal to 1×10⁻³ Ω·cm and lower than 1×10⁴ Ω·cm, more preferably higher than or equal to 1×10⁻³ Ω·cm and lower than 1×10⁻¹ Ω·cm. Note that the insulating layer 314 also has an advantageous effect of preventing an external impurity such as water, alkali metal, or alkaline earth metal, from diffusing into the oxide semiconductor layer 308 a included in the transistor 102.

For example, the insulating layer 314 can be formed using a silicon nitride film, a silicon nitride oxide film, or the like having a thickness in the range from 50 nm to 400 nm. In this embodiment, the insulating layer 314 is formed using a silicon nitride film having a thickness of 100 nm.

The silicon nitride film is preferably formed at a high temperature to have an improved blocking property; for example, the silicon nitride film is preferably formed at a temperature in the range from the substrate temperature of 100° C. to the strain point of the substrate, more preferably at a temperature in the range from 300° C. to 400° C. When the silicon nitride film is formed at a high temperature, a phenomenon in which oxygen is released from the oxide semiconductor layer 308 a and the carrier concentration is increased is caused in some cases; therefore, the upper limit of the temperature is a temperature at which the phenomenon is not caused.

The capacitor 105 is formed concurrently with the oxide semiconductor layer 308 b. The capacitor 105 includes a dielectric layer between a pair of electrodes. One of the pair of electrodes is formed using the conductive layer 304 b, and the other electrode is formed using the oxide semiconductor layer 308 b. The insulating layers 305 and 306 serve as the dielectric layer of the capacitor 105.

Next, the insulating layer 314 is processed into a desired shape, so that an opening 364 is formed (see FIG. 4C).

The opening 364 is formed so as to expose the oxide semiconductor layer 308 b. An example of a formation method of the opening 364 includes, but not limited to, a dry etching method. Alternatively, a wet etching method or a combination of dry etching and wet etching can be employed for formation of the opening 364. The etching step for forming the opening 364 reduces the thickness of the oxide semiconductor layer 308 b in some cases.

The oxide semiconductor layer 308 b that is exposed by the formation of the opening 364 serves as the pixel electrode and the one electrode of the capacitor.

Through the above steps, the transistor 102, the capacitor 105, and the pixel electrode can be formed over the same substrate.

Next, a structure that is formed below the substrate 342 provided so as to face the substrate 302 will be described below.

A film having a coloring property (hereinafter referred to as a coloring layer 346) is formed below the substrate 342. The coloring layer 346 serves as a color filter. Furthermore, a light-blocking layer 344 adjacent to the coloring layer 346 is formed below the lower surface of the substrate 342. The light-blocking layer 344 serves as a black matrix. The coloring layer 346 is not necessarily provided in the case where the display device is a monochrome display device, for example.

The coloring layer 346 is formed using a coloring film that can transmit light in a specific wavelength range. For example, a red (R) color filter for transmitting light in a red wavelength range, a green (G) color filter for transmitting light in a green wavelength range, a blue (B) color filter for transmitting light in a blue wavelength range, or the like can be used.

The light-blocking layer 344 has a function of blocking light in a particular wavelength range, and can be formed using a metal film, an organic insulating film including a black pigment, or the like.

An insulating layer 348 is formed below the coloring layer 346. The insulating layer 348 has a function of a planarization layer or a function of suppressing diffusion of impurities in the coloring layer 346 to the liquid crystal element side.

Next, a method for forming the structure below the substrate 342 provided so as to face the substrate 302 will be described below.

First, the substrate 342 is prepared. For materials of the substrate 342, the materials that can be used for the substrate 302 can be referred to. Then, the light-blocking layer 344 and the coloring layer 346 each are formed below the substrate 342 in a desired position with the use of any of various materials by a printing method, an inkjet method, an etching method using a photolithography technique, or the like.

Then, the insulating layer 348 is formed below the light-blocking layer 344 and the coloring layer 346. For the insulating layer 348, an organic insulating film of an acrylic resin or the like can be used. The insulating layer 348 can prevent an impurity or the like contained in the coloring layer 346 from diffusing to the liquid crystal layer 320 side, for example. Note that the insulating layer 348 is not necessarily formed.

Then, the conductive layer 350 is formed below the insulating layer 348. The conductive layer 350 can be formed using a light-transmitting conductive material such as indium oxide containing tungsten oxide, indium zinc oxide containing tungsten oxide, indium oxide containing titanium oxide, indium tin oxide containing titanium oxide, indium tin oxide (hereinafter referred to as ITO), indium zinc oxide, or indium tin oxide to which silicon oxide is added. The conductive layer 350 can be formed by a sputtering method, for example.

Through the above steps, the structure below the substrate 342 can be formed.

Next, the alignment film 318 is formed over the substrate 302, specifically, over the insulating layer 314 and the oxide semiconductor layer 308 b over the substrate 302. Furthermore, the alignment film 352 is formed below the substrate 342, specifically, below the conductive layer 350 below the substrate 342. The alignment films 318 and 352 can be formed by a rubbing method, an optical alignment method, or the like. After that, the liquid crystal layer 320 is formed between the substrates 302 and 342. The liquid crystal layer 320 can be formed by a dispenser method (a dropping method), or an injecting method in which liquid crystal is injected using a capillary action after the substrates 302 and 342 are bonded to each other.

Through the above steps, the display device illustrated in FIG. 2 can be fabricated.

Modification Example 1

Next, modification examples of the transistor 102, the capacitor 105, and a pixel electrode that are included in a display device will be described with reference to FIG. 5. FIG. 5 is a cross-sectional view illustrating a modification example of the display device in FIG. 2. Portions similar to those in FIG. 2 and portions having functions similar to those of portions in FIG. 2 are denoted by the same reference numerals, and the detailed descriptions thereof are omitted.

The display device in FIG. 5 is different from the display device in FIG. 2 in that an opening 366 is formed instead of the opening 362.

The opening 366 can be formed by etching the insulating layers 312 and 314 so that the oxide semiconductor layer 308 b is exposed, after the insulating layers 312 and 314 are successively formed. Specifically, an example of a method for manufacturing the display device illustrated in FIG. 5 will be described with reference to FIGS. 3A to 3C and FIGS. 6A to 6C.

Method for Manufacturing Display Device (Modification Example)

As in the manufacturing method described above, the conductive layers 310 a and 310 b and the insulating layer 312 are formed over the oxide semiconductor layers 308 a and 308 c (see FIG. 3C).

Then, the insulating layer 314 is formed over the insulating layer 312 (see FIG. 6A).

The insulating layers 312 and 314 are preferably formed successively in vacuum, in which case entry of impurities between the insulating layers 312 and 314 can be inhibited.

Next, the insulating layers 312 and 314 are processed into desired shapes, so that the opening 366 is formed (see FIG. 6B).

The opening 366 is formed so as to expose the oxide semiconductor layer 308 c. An example of a formation method of the opening 366 includes, but not limited to, a dry etching method. Alternatively, a wet etching method or a combination of dry etching and wet etching can be employed for formation of the opening 366. The etching step for forming the opening 366 reduces the thickness of the oxide semiconductor layer 308 c in some cases.

After that, heat treatment is preferably performed. By the heat treatment, part of oxygen contained in the insulating layer 312 can be transferred to the oxide semiconductor layer 308 a, so that oxygen vacancies in the oxide semiconductor layer 308 a can be filled. Consequently, the amount of oxygen vacancies in the oxide semiconductor layer 308 a can be reduced, while oxygen vacancies in the oxide semiconductor layer 308 c that is not in contact with the insulating layer 312 are not reduced. Thus, the amount of oxygen vacancies in the oxide semiconductor layer 308 c is larger than that in the oxide semiconductor layer 308 a. The heat treatment can be performed under conditions similar to those for the heat treatment performed after the formation of the oxide semiconductor layers 308 a and 308 c.

Next, plasma treatment is performed on the oxide semiconductor layer 308 c. Specifically, plasma treatment is performed on the oxide semiconductor layer 308 c and the insulating layer 314, so that the oxide semiconductor layer 308 c becomes the oxide semiconductor layer 308 b having an increased amount of oxygen vacancies and/or an increased impurity concentration (see FIG. 6C).

The arrows in FIG. 6C schematically denote the plasma treatment. The vicinity of surfaces of the oxide semiconductor layer 308 b and the insulating layer 314 is irradiated with plasma. As the plasma treatment, plasma treatment using a gas containing one of a rare gas (He, Ne, Ar, Kr, or Xe), hydrogen, and nitrogen is typical. Specifically, plasma treatment in an Ar atmosphere, plasma treatment in a mixed gas atmosphere of Ar and hydrogen, plasma treatment in an ammonia atmosphere, plasma treatment in a mixed gas atmosphere of Ar and ammonia, plasma treatment in a nitrogen atmosphere, or the like can be employed.

By the above plasma treatment, the amount of oxygen vacancies in the oxide semiconductor layer 308 c is increased because of plasma damage or gas species used in the plasma treatment is introduced into the oxide semiconductor layer 308 c, the resistance of the oxide semiconductor layer 308 c is reduced. Accordingly, the oxide semiconductor layer 308 c becomes the oxide semiconductor layer 308 b.

Note that a plasma CVD apparatus, an ashing apparatus, a sputtering apparatus, an etching apparatus, or the like can be used for the plasma treatment. In this embodiment, the plasma treatment is performed using a plasma CVD apparatus in an Ar atmosphere under the following conditions; the treatment pressure is 200 Pa; the power of an RF power source in the 27 MHz band is 1 kW; the treatment time is 300 sec; and the treatment temperature is 350° C.

Note that a surface of the oxide semiconductor layer 308 a is protected by the insulating layers 312 and 314 when the plasma treatment illustrated in FIG. 6C is performed. Thus, the surface of the oxide semiconductor layer 308 a is not exposed to plasma in the plasma treatment, and an increase in oxygen vacancies and/or hydrogen concentration can be thus inhibited. In other words, the oxide semiconductor layer 308 a can be i-type or substantially i-type.

Modification Example 2

Next, in a display device illustrated in FIG. 7A, the oxide semiconductor layer 308 a used for the transistor 102 in the display device has a layered structure of an oxide semiconductor layer 307 a and an oxide semiconductor layer 309 a. Further, in the display device illustrated in FIG. 7A, the oxide semiconductor layer 308 a used for the transistor 102 in the display device has a layered structure of an oxide semiconductor layer 307 a and an oxide semiconductor layer 309 a. Thus, the other components are the same as those of the display device illustrated in FIG. 2; hence, the above description can be referred to.

A metal oxide of the oxide semiconductor layers 307 a and 307 b (in this specification below, also referred to as the oxide semiconductor layer 307) and a metal oxide of the oxide semiconductor layers 309 a and 309 b (in this specification below, also referred to as an oxide semiconductor layer 309) preferably have at least one metal element in common. Furthermore, the constituent elements of the oxide semiconductor layer 307 and the oxide semiconductor layer 309 may be the same and the compositions of the constituent elements of the oxide semiconductor layer 307 and the oxide semiconductor layer 309 may be different from each other.

In the case where the oxide semiconductor layer 307 is an In—M—Zn oxide (M represents Al, Ti, Ga, Y, Zr, La, Ce, Nd or Hf), it is preferable that the atomic ratio of metal elements of a sputtering target used for forming a film of the In—M—Zn oxide satisfy In≧M and Zn≧M. As the atomic ratio of metal elements of the sputtering target, In:M:Zn=1:1:1, In:M:Zn=5:5:6 (1:1:1.2), and In:M:Zn=3:1:2 are preferable. Note that the atomic ratio of metal elements in the oxide semiconductor layer 307 formed using the above sputtering target varies from that of the metal elements in the above sputtering target within a range of ±20% as an error.

In the case of using an In—M—Zn oxide for the oxide semiconductor layer 307, when Zn and O are eliminated from consideration, the proportion of In and the proportion of M are preferably greater than or equal to 25 atomic % and less than 75 atomic %, respectively, more preferably greater than or equal to 34 atomic % and less than 66 atomic %, respectively.

The energy gap of the oxide semiconductor layer 307 is 2 eV or more, preferably 2.5 eV or more, more preferably 3 eV or more. The off-state current of the transistor 102 can be reduced by using an oxide semiconductor having such a wide energy gap.

The thickness of the oxide semiconductor layer 307 is greater than or equal to 3 nm and less than or equal to 200 nm, preferably greater than or equal to 3 nm and less than or equal to 100 nm, more preferably greater than or equal to 3 nm and less than or equal to 50 nm.

The oxide semiconductor layer 309 is typically an In—Ga oxide, an In—Zn oxide, or an In—M—Zn oxide (M represents Al, Ti, Ga, Y, Zr, La, Ce, Nd or Hf). The energy of the bottom of the conduction band in the oxide semiconductor layer 309 is closer to a vacuum level than that of the oxide semiconductor layer 307 is, and typically, the difference between the energy of the bottom of the conduction band in the oxide semiconductor layer 309 and the energy of the bottom of the conduction band in the oxide semiconductor layer 307 is 0.05 eV or more, 0.07 eV or more, or 0.1 eV or more, and 0.15 eV or more, and 2 eV or less, 1 eV or less, 0.5 eV or less, or 0.4 eV or less. That is, the difference between the electron affinity of the oxide semiconductor layer 309 and the electron affinity of the oxide semiconductor layer 307 is 0.05 eV or more, 0.07 eV or more, 0.1 eV or more, or 0.15 eV or more, and 2 eV or less, 1 eV or less, 0.5 eV or less, or 0.4 eV or less.

The oxide semiconductor layer 309 containing a larger amount of the element M in an atomic ratio than the amount of In in an atomic ratio may have any of the following effects:

(1) the energy gap of the oxide semiconductor layer 309 is wide; (2) the electron affinity of the oxide semiconductor layer 309 is low; (3) an impurity from the outside is blocked; (4) the insulating property is more excellent than that of the oxide semiconductor layer 307. Furthermore, oxygen vacancies are less likely to be generated in the oxide semiconductor layer 309 containing a larger amount of M in an atomic ratio than the amount of In in an atomic ratio because M is a metal element strongly bonded to oxygen.

In the case of using an In—M—Zn oxide for the oxide semiconductor layer 309, when Zn and O are eliminated from consideration, the proportion of In and the proportion of M are preferably less than 50 atomic % and greater than or equal to 50 atomic %, respectively, more preferably less than 25 atomic % and greater than or equal to 75 atomic %, respectively.

In the case where each of the oxide semiconductor layer 307 and the oxide semiconductor layer 309 is In—M—Zn oxide (M represents Al, Ti, Ga, Y, Zr, La, Ce, Nd or Hf), the proportion of M atoms in the oxide semiconductor layer 309 is higher than the proportion of M atoms in the oxide semiconductor layer 307. Typically, the proportion of M atoms in the oxide semiconductor layer 309 is more than or equal to 1.5 times, preferably more than or equal to 2 times, more preferably more than or equal to 3 times as high as that in the oxide semiconductor layer 307.

In the case where the oxide semiconductor layer 309 has an atomic ratio of In to M and Zn that is x₁:y₁:z₁ and the oxide semiconductor layer 307 has an atomic ratio of In to M and Zn that is x₂:y₂:z₂, y₁/x₁ is larger than y₂/x₂, preferably y₁/x₁ is more than or equal to 1.5 times as large as y₂/x₂, more preferably y₁/x₁ is more than or equal to 2 times as large as y₂/x₂, still more preferably y₁/x₁ is more than or equal to three times as large as y₂/x₂. In this case, it is preferable that in the oxide semiconductor layer, y₂ be higher than or equal to x₂ because a transistor including the oxide semiconductor film can have stable electric characteristics. In this case, it is preferable that in the oxide semiconductor layer, y₂ be higher than or equal to x₂ because the transistor 102 including the oxide semiconductor layer can have stable electric characteristics. However, when y₂ is higher than or equal to three times x₂, the field-effect mobility of the transistor 102 including the oxide semiconductor layer is reduced. Thus, it is preferable that y₂ be lower than three times x₂.

In the case where the oxide semiconductor layer 309 is an In—M—Zn oxide, it is preferable that the atomic ratio of metal elements of a sputtering target used for forming a film of the In—M—Zn oxide satisfy M>In and Zn≧M. As the atomic ratio of metal elements of the sputtering target, In:Ga:Zn=1:3:2, In:Ga:Zn=1:3:3, In:Ga:Zn=1:3:4, In:Ga:Zn=1:3:5, In:Ga:Zn=1:3:6, In:Ga:Zn=1:3:7, In:Ga:Zn=1:3:8, In:Ga:Zn=1:3:9, In:Ga:Zn=1:3:10, In:Ga:Zn=1:6:4, In:Ga:Zn=1:6:5, In:Ga:Zn=1:6:6, In:Ga:Zn=1:6:7, In:Ga:Zn=1:6:8, In:Ga:Zn=1:6:9, and In:Ga:Zn=1:6:10 are preferable. Note that the atomic ratio of metal elements in each of the oxide semiconductor layers 307 and 309 formed using the above sputtering target varies from that of the metal elements in the above sputtering target within a range of ±20% as an error.

Note that the composition is not limited to those described above, and a material having the appropriate composition may be used depending on needed semiconductor characteristics and electrical characteristics (e.g., field-effect mobility and threshold voltage) of the transistor. In order to obtain the required semiconductor characteristics of the transistor, it is preferable that the carrier density, the impurity concentration, the defect density, the atomic ratio of a metal element to oxygen, the interatomic distance, the density, and the like of the oxide semiconductor layer 307 be set to appropriate values.

The oxide semiconductor layer 309 also functions as a film that relieves damage to the oxide semiconductor layer 307 at the time of forming the insulating layers 312 and 314 later. The thickness of the oxide semiconductor layer 309 is greater than or equal to 3 nm and less than or equal to 100 nm, preferably greater than or equal to 3 nm and less than or equal to 50 nm.

When silicon or carbon, which is one of Group 14 elements, is contained in the oxide semiconductor layer 307 a in the transistor 102, the amount of oxygen vacancies is increased, and the oxide semiconductor layer 307 a becomes n-type. Thus, the concentration of silicon or carbon (the concentration is measured by SIMS) in the oxide semiconductor layer 307 a or the concentration of silicon or carbon (the concentration is measured by SIMS) in the vicinity of the interface between the oxide semiconductor layer 309 a and the oxide semiconductor layer 307 a is set to be lower than or equal to 2×10¹⁸ atoms/cm³, preferably lower than or equal to 2×10¹⁷ atoms/cm³.

Furthermore, the concentration of alkali metal or alkaline earth metal of the oxide semiconductor layer 307 a, which is measured by SIMS, is lower than or equal to 1×10¹⁸ atoms/cm³, preferably lower than or equal to 2×10¹⁶ atoms/cm³. Alkali metal and alkaline earth metal might generate carriers when bonded to an oxide semiconductor, in which case the off-state current of the transistor might be increased. Therefore, it is preferable to reduce the concentration of alkali metal or alkaline earth metal in the oxide semiconductor layer 307 a.

When containing nitrogen, the oxide semiconductor layer 307 a easily becomes n-type because of generation of electrons serving as carriers and an increase in carrier density. Thus, a transistor including an oxide semiconductor that contains nitrogen is likely to be normally on. For this reason, nitrogen in the oxide semiconductor film is preferably reduced as much as possible; the nitrogen concentration measured by SIMS is preferably set to, for example, lower than or equal to 5×10¹⁸ atoms/cm³.

Note that in the transistor 102 illustrated in FIG. 7A, the oxide semiconductor layer 309 a is provided between the oxide semiconductor layer 307 a and the insulating layer 312. The oxide semiconductor layer 307 a is positioned on the conductive layer 304 a side (the conductive layer 304 a serves as a gate electrode) and serves as a main path of carriers. Hence, if trap states are formed between the oxide semiconductor layer 309 a and the insulating layer 312 owing to impurities and defects, electrons flowing in the oxide semiconductor layer 307 a are less likely to be captured by the trap states because there is a distance between the trap states and the oxide semiconductor layer 307 a. Accordingly, the amount of on-state current of the transistor 102 can be increased, and the field-effect mobility can be increased. When the electrons are captured by the trap states, the electrons become negative fixed charge. Consequently, the threshold voltage of the transistor 102 is changed. However, the distance between the oxide semiconductor layer 307 a and the trap states enables reducing capture of the electrons by the trap states, and accordingly a change in the threshold voltage can be reduced.

Note that the oxide semiconductor layers 307 a to 309 a are not formed by simply stacking layers but are formed to have a continuous energy band (here, in particular, a structure in which energies of the bottoms of the conduction bands are changed continuously between the layers). In other words, the oxide semiconductor layers have a layered structure such that there exists no impurities that forms a defect level such as a trap center or a recombination center at each interface. If an impurity exists between the stacked oxide semiconductor layers 307 a and 309 a, continuity of the energy band is lost, and thus carriers are trapped or disappear by recombination at the interface.

In order to form such a continuous energy band, layers need to be successively formed without being exposed to the air, with use of a multi-chamber deposition apparatus (sputtering apparatus) including a load lock chamber. Each chamber in the sputtering apparatus is preferably evacuated to be a high vacuum (to the degree of approximately 5×10⁻⁷ Pa to 1×10⁻⁴ Pa) by an entrapment vacuum pump such as a cryopump so that water and the like as impurities of the oxide semiconductor layer are removed as much as possible. Alternatively, a turbo molecular pump and a cold trap are preferably combined so as to prevent a backflow of a gas, especially a gas containing carbon or hydrogen, from an exhaust system to the inside of the chamber.

Here, a band structure of the layered structure in the transistor 102 will be described with reference to FIG. 7B.

FIG. 7B schematically shows part of a band structure in the transistor 102. Here, the case where silicon oxide layers are provided as the insulating layers 306 and 312 is shown. In FIG. 7B, EcI1 denotes the energy of the bottom of the conduction band in the silicon oxide layer that is used as the insulating layer 306; EcS1 denotes the energy of the bottom of the conduction band in the oxide semiconductor layer 307 a; EcS2 denotes the energy of the bottom of the conduction band in the oxide semiconductor layer 309 a; and EcI2 denotes the energy of the bottom of the conduction band in the silicon oxide layer that is used as the insulating layer 312.

As shown in FIG. 7B, there is no energy barrier between the oxide semiconductor layers 307 a and 309 a, and the energy level of the bottom of the conduction band is changed smoothly, or continuously. This is because the oxide semiconductor layers 307 a and 309 a contain a common element and oxygen is transferred between the oxide semiconductor layers 307 a and 309 a, so that a mixed layer is formed.

As shown in FIG. 7B, the oxide semiconductor layer 307 a in the oxide semiconductor layer 308 a serves as a well and a channel region of the transistor including the oxide semiconductor layer 308 a is formed in the oxide semiconductor layer 307 a. Note that since the energy of the bottom of the conduction band of the oxide semiconductor layer 308 a is continuously changed, it can be said that the oxide semiconductor layers 307 a and 309 a are continuous.

Although trap levels due to impurities or defects might be formed in the vicinity of the interface between the oxide semiconductor layer 309 a and the insulating layer 312 as shown in FIG. 7B, the oxide semiconductor layer 307 a can be distanced from the trap levels owing to existence of the oxide semiconductor layer 309 a. However, in the case where an energy difference between EcS1 and EcS2 is small, electrons in the oxide semiconductor layer 307 a might reach the trap state by passing over the energy gap. When the electrons are captured by the trap level, negative fixed charge is generated at the interface with the insulating layer 312, whereby the threshold voltage of the transistor shifts in the positive direction. Therefore, it is preferable that the energy difference between EcS1 and EcS2 be 0.1 eV or more, more preferably 0.15 eV or more because a change in the threshold voltage of the transistor is prevented and stable electrical characteristics are obtained.

In the structure illustrated in FIG. 7A, one electrode of the capacitor 105 and the pixel electrode each have a layered structure of the oxide semiconductor layers 307 b and 309 b. Hydrogen diffusion from the insulating film in contact with the oxide semiconductor layer 309 b or impurity implantation and/or diffusion by plasma treatment improves the conductivity of the layered structure of the oxide semiconductor layers 307 b and 309 b. Thus, the oxide semiconductor layers 307 b and 309 b serve as the one electrode of the capacitor 105 and the pixel electrode.

Modification Example 3

Next, a structural example of a display device of one embodiment of the present invention in which a light-emitting element is used as a display element will be described with reference to FIG. 8. Note that FIG. 8 is a cross-sectional view of a modification example of the display device illustrated in FIG. 2; thus, portions similar to those in FIG. 2 and portions having functions similar to those of portions in FIG. 2 are denoted by the same reference numerals, and the detailed descriptions thereof are omitted.

The display device in FIG. 8 is different from the display device in FIG. 2 in that a light-emitting element 370 is provided instead of the liquid crystal element 322. The light-emitting element 370 has a light-emitting layer between a pair of electrodes. Specifically, the light-emitting element 370 includes the oxide semiconductor layer 308 b serving as one of the pair of electrodes and the pixel electrode, an electrode 374 serving as the other electrode, and a light-emitting layer 372 interposed between the pair of electrodes.

As the light-emitting element 370, a light-emitting element utilizing electroluminescence can be used. Light-emitting elements utilizing electroluminescence are classified according to whether a light-emitting material is an organic compound or an inorganic compound. In general, the former is referred to as an organic EL element, and the latter as an inorganic EL element. A structure in FIG. 8 where an organic EL element is used as the light-emitting element 370 will be described below.

In an organic EL element, by application of a voltage to a light-emitting element, electrons and holes are separately injected from a pair of electrodes into a layer containing a light-emitting organic compound, and a current flows. The electrons and holes (i.e., carriers) are recombined; thus, the light-emitting organic compound becomes in an excited state. The light-emitting organic compound returns to a ground state from the excited state, thereby emitting light. Based on such a mechanism, such a light-emitting element is referred to as a current-excitation type light-emitting element.

To extract light emitted from the light-emitting element, it is necessary that at least one of the pair of electrodes has a light-transmitting property. A transistor and the light-emitting element are formed over a substrate. Any of light-emitting elements having the following structures can be used as the light-emitting element: a top-emission structure in which light is extracted through the surface opposite to the substrate, a bottom-emission structure in which light is extracted through the surface of the substrate, and a dual-emission structure in which light is extracted through the surface opposite to the substrate and the surface of the substrate.

The display device in FIG. 8 is what is called a bottom-emission display device in which light can be extracted from the substrate 302 side. Thus, the oxide semiconductor layer 308 b serving as one of the pair of electrodes of the light-emitting element 370 transmits light. The electrode 374 serving as the other electrode of the light-emitting element 370 reflects light.

The light-emitting element 370 serving as a display element is electrically connected to the transistor 102. Although the light-emitting element 370 in FIG. 8 has a layered structure of the oxide semiconductor layer 308 b serving as the pixel electrode, the light-emitting layer 372, and the electrode 374, the structure of the light-emitting element 370 is not limited to this structure. For example, the structure of the light-emitting element 370 can be changed as appropriate depending on, for example, the direction in which light is extracted from the light-emitting layer 372.

The light-emitting layer 372 can be formed to have either a single-layer structure or layered structure including a plurality of layers.

A protective film may be formed over the electrode 374 in order to prevent entry of oxygen, hydrogen, moisture, carbon dioxide, or the like into the light-emitting element 370. As the protective film, a silicon nitride film, a silicon nitride oxide film, an aluminum oxide film, or the like can be formed. In addition, a space formed between the substrates 302 and 342 is provided with a filler 376 and sealed. It is preferable that the light-emitting element be packaged (sealed) with a cover material or a protective film (such as a laminate film or an ultraviolet curable resin film) with high air-tightness and little degasification so that the light-emitting element is not exposed to the outside air, in this manner.

Furthermore, in the display device in FIG. 8, a partition 368 is provided over the insulating layer 314 so as to cover an outer portion of the oxide semiconductor layer 308 b.

The partition 368 can be formed using an organic resin or an inorganic insulating material. As the organic resin, for example, a polyimide resin, a polyamide resin, an acrylic resin, a siloxane resin, an epoxy resin, a phenol resin, or the like can be used. As the inorganic insulating material, silicon oxide, silicon oxynitride, or the like can be used. In particular, a photosensitive resin is preferably used to facilitate formation of the partition 368. Although FIG. 8 illustrates the structure where the partition 368 is provided, the structure is not necessarily employed. For example, a structure without the partition 368 may be employed.

The electrode 374 can be formed using a reflective conductive film, for example. The electrode 374 can be formed using one or more materials selected from metals such as tungsten (W), molybdenum (Mo), zirconium (Zr), hafnium (Hf), vanadium (V), niobium (Nb), tantalum (Ta), chromium (Cr), cobalt (Co), nickel (Ni), titanium (Ti), platinum (Pt), aluminum (Al), copper (Cu), and silver (Ag); an alloy of any of these metals; and a nitride of any of these metals.

For the filler 376, an inert gas such as nitrogen or argon, or an ultraviolet curable resin or a thermosetting resin can be used. For example, a polyvinyl chloride (PVC) resin, an acrylic resin, a polyimide resin, an epoxy resin, a silicone resin, a polyvinyl butyral (PVB) resin, or an ethylene vinyl acetate (EVA) resin can be used for the filler. For example, nitrogen may be used for the filler.

In addition, if needed, an optical film, such as a polarizing plate, a circularly polarizing plate (including an elliptically polarizing plate), a retardation plate (a quarter-wave plate or a half-wave plate), or a color filter, may be provided as appropriate on a light-emitting surface of the light-emitting element. Further, the polarizing plate or the circularly polarizing plate may be provided with an anti-reflection film. For example, anti-glare treatment by which reflected light can be diffused by projections and depressions on the surface so as to reduce the glare can be performed.

As to the display device described in this embodiment, it is possible to simultaneously form the oxide semiconductor layer 308 a serving as the channel formation region of the transistor and the oxide semiconductor layer 308 b serving as the electrode of the capacitor and the pixel electrode. The oxide semiconductor layer 308 a is in contact with layers each formed using a material capable of improving the property of the interface with the oxide semiconductor layer, such as the insulating layers 306 and 312; accordingly, the oxide semiconductor layer 308 a serves as a semiconductor. Therefore, the transistor including the oxide semiconductor layer 308 a has excellent electric characteristics. On the other hand, an impurity is implanted into the oxide semiconductor layer 308 b serving as the pixel electrode and the one electrode of the capacitor by plasma treatment in an atmosphere containing hydrogen or diffusion of hydrogen from the insulating layer containing hydrogen. Thus, when hydrogen is diffused to the oxide semiconductor layer 308 b, hydrogen is bonded to oxygen and electrons serving as carriers are generated in the oxide semiconductor layer 308 b. Accordingly, the oxide semiconductor layer 308 b can have an increased conductivity and serves as a conductor. That is, the oxide semiconductor layer 308 b can also be referred to as a highly conductive oxide semiconductor layer.

Further, as to the display device described in this embodiment, the one electrode of the capacitor is formed at the same time as the gate electrode of the transistor. In addition, the other electrode of the capacitor is formed at the same time as the oxide semiconductor layer of the transistor. Note that the other electrode of the capacitor also serves as the pixel electrode. The dielectric layer of the capacitor is formed at the same time as the gate insulating layer of the transistor. Thus, a step of further forming a conductive layer and/or a dielectric layer to form the capacitor and the pixel electrode is unnecessary, reducing the manufacturing steps of the display device. This enables the display device to be manufactured at low cost.

The structures, the methods, and the like described in this embodiment can be combined as appropriate with any of the structures, the methods, and the like described in the other embodiments.

Embodiment 2

In this embodiment, an example of an oxide semiconductor layer that can be used for the transistor, the electrode of the capacitor, and the pixel electrode described in Embodiment 1 will be described.

<Crystallinity of Oxide Semiconductor Layer>

A structure of an oxide semiconductor layer will be described below.

An oxide semiconductor layer is classified roughly into a single-crystal oxide semiconductor layer and a non-single-crystal oxide semiconductor layer. The non-single-crystal oxide semiconductor layer includes any of an amorphous oxide semiconductor layer, a microcrystalline oxide semiconductor layer, a polycrystalline oxide semiconductor layer, a c-axis aligned crystalline oxide semiconductor (CAAC-OS) film, and the like.

First, a CAAC-OS film will be described.

The CAAC-OS film is one of oxide semiconductor films including a plurality of crystal parts, and most of the crystal parts each fit inside a cube whose one side is less than 100 nm. Thus, there is a case where a crystal part included in the CAAC-OS film fits inside a cube whose one side is less than 10 nm, less than 5 nm, or less than 3 nm.

In a transmission electron microscope (TEM) image of the CAAC-OS film, a boundary between crystal parts, that is, a grain boundary is not clearly observed. Thus, in the CAAC-OS film, a reduction in electron mobility due to the grain boundary is less likely to occur.

According to the TEM image of the CAAC-OS film observed in a direction substantially parallel to a sample surface (cross-sectional TEM image), metal atoms are arranged in a layered manner in the crystal parts. Each metal atom layer has a morphology reflected by a surface over which the CAAC-OS film is formed (hereinafter, a surface over which the CAAC-OS film is formed is referred to as a formation surface) or a top surface of the CAAC-OS film, and is arranged in parallel to the formation surface or the top surface of the CAAC-OS film.

In this specification and the like, a term “parallel” indicates that the angle formed between two straight lines is greater than or equal to −10° and less than or equal to 10°, and accordingly also includes the case where the angle is greater than or equal to −5° and less than or equal to 5°. In addition, a term “perpendicular” indicates that the angle formed between two straight lines is greater than or equal to 80° and less than or equal to 100°, and accordingly includes the case where the angle is greater than or equal to 85° and less than or equal to 95°.

On the other hand, according to the TEM image of the CAAC-OS film observed in a direction substantially perpendicular to the sample surface (plan TEM image), metal atoms are arranged in a triangular or hexagonal arrangement in the crystal parts. However, there is no regularity of arrangement of metal atoms between different crystal parts.

From the results of the cross-sectional TEM image and the plan TEM image, alignment is found in the crystal parts in the CAAC-OS film.

A CAAC-OS film is subjected to structural analysis with an X-ray diffraction (XRD) apparatus. For example, when the CAAC-OS film including an InGaZnO₄ crystal is analyzed by an out-of-plane method, a peak appears frequently when the diffraction angle (2θ) is around 31°. This peak is derived from the (009) plane of the InGaZnO₄ crystal, which indicates that crystals in the CAAC-OS film have c-axis alignment, and that the c-axes are aligned in a direction substantially perpendicular to the formation surface or the top surface of the CAAC-OS film.

On the other hand, when the CAAC-OS film is analyzed by an in-plane method in which an X-ray enters a sample in a direction substantially perpendicular to the c-axis, a peak appears frequently when 2θ is around 56°. This peak is derived from the (110) plane of the InGaZnO₄ crystal. Here, analysis (φ scan) is performed under conditions where the sample is rotated around a normal vector of a sample surface as an axis (φ axis) with 2θ fixed at around 56°. In the case where the sample is a single-crystal oxide semiconductor film of InGaZnO₄, six peaks appear. The six peaks are derived from crystal planes equivalent to the (110) plane. On the other hand, in the case of a CAAC-OS film, a peak is not clearly observed even when φ scan is performed with 2θ fixed at around 56°.

According to the above results, in the CAAC-OS film having c-axis alignment, while the directions of a-axes and b-axes are different between crystal parts, the c-axes are aligned in a direction parallel to a normal vector of a formation surface or a normal vector of a top surface. Thus, each metal atom layer arranged in a layered manner observed in the cross-sectional TEM image corresponds to a plane parallel to the a-b plane of the crystal.

Note that the crystal part is formed concurrently with deposition of the CAAC-OS film or is formed through crystallization treatment such as heat treatment. As described above, the c-axis of the crystal is aligned in a direction parallel to a normal vector of a formation surface or a normal vector of a top surface. Thus, for example, in the case where a shape of the CAAC-OS film is changed by etching or the like, the c-axis might not be necessarily parallel to a normal vector of a formation surface or a normal vector of a top surface of the CAAC-OS film.

Further, the degree of crystallinity in the CAAC-OS film is not necessarily uniform. For example, in the case where crystal growth leading to the CAAC-OS film occurs from the vicinity of the top surface of the film, the degree of the crystallinity in the vicinity of the top surface is higher than that in the vicinity of the formation surface in some cases. Further, when an impurity is added to the CAAC-OS film, the crystallinity in a region to which the impurity is added is changed, and the degree of crystallinity in the CAAC-OS film varies depending on regions.

Note that when the CAAC-OS film with an InGaZnO₄ crystal is analyzed by an out-of-plane method, a peak of 2θ may also be observed at around 36°, in addition to the peak of 2θ at around 31°. The peak of 2θ at around 36° indicates that a crystal having no c-axis alignment is included in part of the CAAC-OS film. It is preferable that in the CAAC-OS film, a peak of 2θ appears at around 31° and a peak of 2θ does not appear at around 36°.

In this specification, the trigonal and rhombohedral crystal systems are included in the hexagonal crystal system.

The CAAC-OS film is an oxide semiconductor layer having a low impurity concentration. The impurity is an element other than the main components of the oxide semiconductor layer, such as hydrogen, carbon, silicon, and a transition metal element. In particular, an element that has higher bonding strength to oxygen than a metal element included in the oxide semiconductor layer, such as silicon, disturbs the atomic arrangement of the oxide semiconductor layer by depriving the oxide semiconductor layer of oxygen and causes a decrease in crystallinity. Further, heavy metals such as iron and nickel, argon, carbon dioxide, and the like each have a large atomic radius (molecular radius), and thus disturb the atomic arrangement of the oxide semiconductor layer and causes a decrease in crystallinity when any of them is contained in the oxide semiconductor layer. Note that the impurity contained in the oxide semiconductor layer might serve as a carrier trap or a carrier generation source.

Further, the CAAC-OS film is an oxide semiconductor layer having a low density of defect states.

In a transistor using the CAAC-OS film, change in electric characteristics due to irradiation with visible light or ultraviolet light is small.

Next, a microcrystalline oxide semiconductor layer will be described.

In an image obtained with TEM, crystal parts cannot be found clearly in the microcrystalline oxide semiconductor layer in some cases. In most cases, the size of a crystal part in the microcrystalline oxide semiconductor is greater than or equal to 1 nm and less than or equal to 100 nm, or greater than or equal to 1 nm and less than or equal to 10 nm. A microcrystal with a size greater than or equal to 1 nm and less than or equal to 10 nm or a size greater than or equal to 1 nm and less than or equal to 3 nm is specifically referred to as nanocrystal (nc). An oxide semiconductor layer including a nanocrystal is referred to as a nanocrystalline oxide semiconductor (nc-OS) film. In an image obtained with TEM, a crystal grain cannot be found clearly in the nc-OS film in some cases.

In the nc-OS film, a microscopic region (for example, a region with a size greater than or equal to 1 nm and less than or equal to 10 nm, in particular, a region with a size greater than or equal to 1 nm and less than or equal to 3 nm) has a periodic atomic order. Further, there is no regularity of crystal orientation between different crystal parts in the nc-OS film; thus, the orientation of the whole film is not observed. Accordingly, in some cases, the nc-OS film cannot be distinguished from an amorphous oxide semiconductor depending on an analysis method. For example, when the nc-OS film is subjected to structural analysis by an out-of-plane method with an XRD apparatus using an X-ray having a diameter larger than a crystal part, a peak which shows a crystal plane does not appear. Further, a halo pattern is shown in a selected-area electron diffraction pattern of the nc-OS film which is obtained by using an electron beam having a probe diameter (e.g., larger than or equal to 50 nm) larger than the diameter of a crystal part. Meanwhile, spots are shown in a nanobeam electron diffraction pattern of the nc-OS film which is obtained by using an electron beam having a probe diameter (e.g., larger than or equal to 1 nm and smaller than or equal to 30 nm) close to, or smaller than or equal to the diameter of a crystal part. Further, in a nanobeam electron diffraction pattern of the nc-OS film, regions with high luminance in a circular (ring) pattern are shown in some cases. Also in a nanobeam electron diffraction pattern of the nc-OS film, a plurality of spots are shown in a ring-like region in some cases.

The nc-OS film is an oxide semiconductor layer having more regularity than the amorphous oxide semiconductor layer; thus, the nc-OS film has a lower density of defect levels than the amorphous oxide semiconductor layer. However, there is no regularity of crystal orientation between different crystal parts in the nc-OS film; hence, the nc-OS film has a higher density of defect states than the CAAC-OS film.

Note that an oxide semiconductor layer may be a stack including two or more of an amorphous oxide semiconductor layer, a microcrystalline oxide semiconductor layer, and a CAAC-OS film, for example.

<Formation Method of CAAC-OS Film>

For example, a CAAC-OS film can be deposited by a sputtering method using a polycrystalline oxide semiconductor sputtering target. When ions collide with the sputtering target, a crystal region included in the sputtering target may be separated from the target along an a-b plane; in other words, sputtered particles each having a plane parallel to an a-b plane (flat-plate-like sputtered particles or pellet-like sputtered particles) may flake off from the sputtering target. In that case, the flat-plate-like or pellet-like sputtered particles reach a substrate while keeping its crystal state, whereby the CAAC-OS film can be formed over the substrate.

The flat-plate-like or pellet-like sputtered particle has, for example, an equivalent circle diameter of a plane parallel to the a-b plane of greater than or equal to 3 nm and less than or equal to 10 nm, and a thickness (a length in the direction perpendicular to the a-b plane) of greater than or equal to 0.7 nm and less than 1 nm. Note that in the flat-plate-like or pellet-like sputtered particle, the plane parallel to the a-b plane may be a regular triangle or a regular hexagon. Here, the term “equivalent circle diameter of a plane” refers to the diameter of a perfect circle having the same area as the plane.

For the deposition of the CAAC-OS film, the following conditions are preferably used.

By increasing the substrate temperature during the deposition, migration of sputtered particles is likely to occur after the sputtered particles reach a substrate surface. Specifically, the substrate temperature during the deposition is higher than or equal to 100° C. and lower than or equal to 740° C., preferably higher than or equal to 200° C. and lower than or equal to 500° C. By increasing the substrate temperature during the deposition, when the flat-plate-like or pellet-like sputtered particles reach the substrate, migration occurs on the substrate surface, so that flat planes of the flat-plate-like sputtered particles are attached to the substrate. At this time, the sputtered particles are charged positively, whereby the sputtered particles are attached to the substrate while repelling each other; thus, the sputtered particles do not overlap with each other randomly, and a CAAC-OS film with a uniform thickness can be deposited.

By reducing the mixing of impurities during the deposition, the crystal state can be prevented from being broken by the impurities. For example, the concentration of impurities (e.g., hydrogen, water, carbon dioxide, or nitrogen) which exist in the deposition chamber may be reduced. Furthermore, the concentration of impurities in a deposition gas may be reduced. Specifically, a deposition gas whose dew point is −80° C. or lower, preferably −100° C. or lower is used.

Furthermore, it is preferable that the proportion of oxygen in the deposition gas be increased and the power be optimized in order to reduce plasma damage at the deposition. The proportion of oxygen in the deposition gas is 30 vol % or higher, preferably 100 vol %.

Alternatively, the CAAC-OS film is formed by the following method.

First, a first oxide semiconductor layer is formed to a thickness of greater than or equal to 1 nm and less than 10 nm. The first oxide semiconductor layer is formed by a sputtering method. Specifically, the substrate temperature is set to higher than or equal to 100° C. and lower than or equal to 500° C., preferably higher than or equal to 150° C. and lower than or equal to 450° C., and the proportion of oxygen in a deposition gas is set to higher than or equal to 30 vol %, preferably 100 vol %.

Then, heat treatment is performed to increase the crystallinity of the first oxide semiconductor layer to give the first CAAC-OS film with high crystallinity. The temperature of the heat treatment is higher than or equal to 350° C. and lower than or equal to 740° C., preferably higher than or equal to 450° C. and lower than or equal to 650° C. The heat treatment time is longer than or equal to 1 minute and shorter than or equal to 24 hours, preferably longer than or equal to 6 minutes and shorter than or equal to 4 hours. The heat treatment is performed in an inert atmosphere or an oxidation atmosphere. It is preferable to perform heat treatment in an inert atmosphere and then perform heat treatment in an oxidation atmosphere. The heat treatment in an inert atmosphere can reduce the concentration of impurities in the first oxide semiconductor layer in a short time. At the same time, the heat treatment in an inert atmosphere may generate oxygen vacancies in the first oxide semiconductor layer. In such a case, the heat treatment in an oxidation atmosphere can reduce the oxygen vacancies. Note that the heat treatment may be performed under a reduced pressure, such as 1000 Pa or lower, 100 Pa or lower, 10 Pa or lower, or 1 Pa or lower. The heat treatment under the reduced pressure can reduce the concentration of impurities in the first oxide semiconductor layer in a shorter time.

The first oxide semiconductor layer with a thickness of greater than or equal to 1 nm and less than 10 nm can be easily crystallized by heat treatment as compared to the case where the first oxide semiconductor layer has a thickness of greater than or equal to 10 nm.

Next, a second oxide semiconductor layer having the same composition as the first oxide semiconductor layer is formed to a thickness of greater than or equal to 10 nm and less than or equal to 50 nm. The second oxide semiconductor layer is formed by a sputtering method. Specifically, the substrate temperature is set to higher than or equal to 100° C. and lower than or equal to 500° C., preferably higher than or equal to 150° C. and lower than or equal to 450° C., and the proportion of oxygen in a deposition gas is set to higher than or equal to 30 vol %, preferably 100 vol %.

Then, heat treatment is performed so that solid phase growth of the second oxide semiconductor layer from the first CAAC-OS film occurs, whereby the second oxide semiconductor film is turned into a second CAAC-OS film having high crystallinity. The temperature of the heat treatment is higher than or equal to 350° C. and lower than or equal to 740° C., preferably higher than or equal to 450° C. and lower than or equal to 650° C. The heat treatment time is longer than or equal to 1 minute and shorter than or equal to 24 hours, preferably longer than or equal to 6 minutes and shorter than or equal to 4 hours. The heat treatment is performed in an inert atmosphere or an oxidation atmosphere. It is preferable to perform heat treatment in an inert atmosphere and then perform heat treatment in an oxidation atmosphere. The heat treatment in an inert atmosphere can reduce the concentration of impurities in the second oxide semiconductor layer in a short time. At the same time, the heat treatment in an inert atmosphere may generate oxygen vacancies in the second oxide semiconductor layer. In such a case, the heat treatment in an oxidation atmosphere can reduce the oxygen vacancies. Note that the heat treatment may be performed under a reduced pressure, such as 1000 Pa or lower, 100 Pa or lower, 10 Pa or lower, or 1 Pa or lower. The heat treatment under the reduced pressure can reduce the concentration of impurities in the second oxide semiconductor layer in a shorter time.

As described above, a CAAC-OS film with a total thickness of greater than or equal to 10 nm can be formed. The CAAC-OS film can be favorably used as the oxide semiconductor layer in the oxide stack.

Next, a method for forming an oxide film in the case where a formation surface has a low temperature (e.g., a temperature lower than 130° C., lower than 100° C., or lower than 70° C., or approximately a room temperature (20° C. to 25° C.)) because, for example, the substrate is not heated will be described.

In the case where the formation surface has a low temperature, sputtered particles fall irregularly to the deposition surface. For example, migration does not occur; therefore, the sputtered particles are randomly deposited on the deposition surface including a region where other sputtered particles have been deposited. That is, an oxide film obtained by the deposition might have a non-uniform thickness and disordered crystal alignment. The oxide film obtained in the above manner maintains the crystallinity of the sputtered particles to a certain degree and thus has a crystal part (nanocrystal).

For example, in the case where the pressure at the deposition is high, the frequency with which the flying sputtered particle collides with another particle (e.g., an atom, a molecule, an ion, or a radical) of argon or the like is increased. When the flying sputtered particle collides with another particle (resputtered), the crystal structure of the sputtered particle might be broken. For example, when the sputtered particle collides with another particle, the plate-like shape of the sputtered particle cannot be kept, and the sputtered particle might be broken into parts (e.g., atomized). In that case, when atoms obtained from the sputtered particle are deposited on the formation surface, an amorphous oxide semiconductor film might be formed.

In the case where not a sputtering method using a target including a polycrystalline oxide but a deposition method using liquid or a method for depositing a film by vaporizing a solid such as a target is used, the atoms separately fly to be deposited on the formation surface; therefore, an amorphous oxide film might be formed. Furthermore, for example, by a laser ablation method, atoms, molecules, ions, radials, clusters, or the like released from the target flies to be deposited on the formation surface; therefore, an amorphous oxide film might be formed.

An oxide semiconductor layer included in the transistor of one embodiment of the present invention may have any of the above crystal states. Furthermore, in the case of stacked oxide semiconductor layers, the crystal states of the oxide semiconductor layers may be different from each other. Note that a CAAC-OS film is preferably used for the oxide semiconductor layer serving as a channel of the transistor. Further, an oxide semiconductor layer included in the capacitor has a higher impurity concentration than the oxide semiconductor layer included in the transistor; thus, the crystallinity is lowered in some cases.

The structures, the methods, and the like described in this embodiment can be combined as appropriate with any of the structures, the methods, and the like described in the other embodiments.

Embodiment 3

In this embodiment, a display device of one embodiment of the present invention will be described with reference to drawings. Note that portions similar to those in the above embodiments and portions having functions similar to those in the above embodiments are given the same reference numerals, and detailed descriptions thereof are omitted.

FIG. 9A illustrates an example of a display device. The display device in FIG. 9A includes a pixel portion 200, a scan line driver circuit 204, a signal line driver circuit 206, m scan lines 207 that are arranged in parallel or substantially in parallel and whose potentials are controlled by the scan line driver circuit 204, and n signal lines 209 that are arranged in parallel or substantially in parallel and whose potentials are controlled by the signal line driver circuit 206. The pixel portion 200 includes a plurality of pixels 301 arranged in a matrix. Capacitor lines 215 that are arranged in parallel or substantially in parallel to the scan lines 207 are also provided. The capacitor lines 215 may be arranged in parallel or substantially in parallel to the signal lines 209. The scan line driver circuit 204 and the signal line driver circuit 206 may be collectively referred to as a driver circuit portion.

Each scan line 207 is electrically connected to the n pixels 301 in the corresponding row among the pixels 202 arranged in m rows and n columns in the pixel portion 200. Each signal line 209 is electrically connected to the m pixels 301 in the corresponding column among the pixels 301 arranged in m rows and n columns. Note that m and n are each an integer of 1 or more. Each capacitor line 215 is electrically connected to the n pixels 301 in the corresponding row among the pixels 301 arranged in m rows and n columns. Note that in the case where the capacitor lines 215 are arranged in parallel or substantially in parallel along the signal lines 209, each capacitor line 215 is electrically connected to the m pixels 301 in the corresponding column among the pixels 301 arranged in m rows and n columns.

The display device described in Embodiment 1 can be used for the pixel 301 illustrated in FIG. 9A.

FIGS. 9B and 9C illustrate circuit configurations that can be used for the pixels 301 in the display device illustrated in FIG. 9A.

The pixel 301 illustrated in FIG. 9B includes the liquid crystal element 322, the transistor 102, and the capacitor 105.

The potential of one of a pair of electrodes of the liquid crystal element 322 is set in accordance with the specifications of the pixel 301 as appropriate. The alignment state of the liquid crystal element 322 depends on written data. A common potential may be supplied to one of the pair of electrodes of the liquid crystal element 322 included in each of the plurality of pixels 301. The potential supplied to one of a pair of electrodes of the liquid crystal element 322 in each of the pixels 301 in one row may be different from the potential supplied to one of a pair of electrodes of the liquid crystal element 322 in each of the pixels 301 in another row.

The liquid crystal element 322 is an element that controls transmission and non-transmission of light by the optical modulation action of liquid crystal. Note that the optical modulation action of a liquid crystal is controlled by an electric field applied to the liquid crystal (including a horizontal electric field, a vertical electric field, and an oblique electric field). Note that any of the following can be used for the liquid crystal element 322: nematic liquid crystal, cholesteric liquid crystal, smectic liquid crystal, discotic liquid crystal, thermotropic liquid crystal, lyotropic liquid crystal, low-molecular liquid crystal, high-molecular liquid crystal, ferroelectric liquid crystal, anti-ferroelectric liquid crystal, main-chain liquid crystal, side-chain high-molecular liquid crystal, a banana-shaped liquid crystal, and the like.

Examples of a driving method of the display device including the liquid crystal element 322 include a TN mode, an STN mode, a VA mode, an axially symmetric aligned micro-cell (ASM) mode, an optically compensated birefringence (OCB) mode, a ferroelectric liquid crystal (FLC) mode, an antiferroelectric liquid crystal (AFLC) mode, an MVA mode, a patterned vertical alignment (PVA) mode, an IPS mode, an FFS mode, and a transverse bend alignment (TBA) mode. Other examples of the driving method of the display device include an electrically controlled birefringence (ECB) mode, a polymer dispersed liquid crystal (PDLC) mode, a polymer network liquid crystal (PNLC) mode, and a guest-host mode. Note that the present invention is not limited to them, and any of various liquid crystal elements and driving methods can be used as a liquid crystal element and a driving method thereof.

The liquid crystal element may be formed using a liquid crystal composition including liquid crystal exhibiting a blue phase and a chiral material. The liquid crystal exhibiting a blue phase has a short response time of 1 msec or less, and is optically isotropic, which makes the alignment process unneeded and the viewing angle dependence small.

In the pixel 301 illustrated in FIG. 9B, one of a source electrode and a drain electrode of the transistor 102 is electrically connected to the signal line 209, and the other is electrically connected to the other of a pair of electrodes of the liquid crystal element 322. A gate electrode of the transistor 102 is electrically connected to the scan line 207. The transistor 102 has a function of controlling whether to write a data signal by being turned on or off.

In the structure of the pixel 301 illustrated in FIG. 9B, one of a pair of electrodes of the capacitor 105 is electrically connected to the capacitor line 215 to which a potential is supplied, and the other is electrically connected to the other of the pair of electrodes of the liquid crystal element 322. The potential of the capacitor line 215 is set in accordance with the specifications of the pixel 301 as appropriate. The capacitor 105 serves as a storage capacitor for storing written data.

For example, in the display device including the pixel 301 in FIG. 9B, the pixels 301 are sequentially selected row by row by the scan line driver circuit 204, whereby the transistors 102 are turned on and a data signal is written.

When the transistors 102 are turned off, the pixels 301 to which the data has been written are brought into a holding state. This operation is performed row by row sequentially; thus, an image can be displayed.

The pixel 301 illustrated in FIG. 9C includes a transistor 216 that performs switching of a display element, the transistor 102 that controls driving of the pixel, a transistor 217, the capacitor 105, and the light-emitting element 370. For example, one embodiment of the display device illustrated in FIG. 8 can be used for the transistor 102 that controls driving of the pixel, the capacitor 105, and the light-emitting element 370.

One of a source electrode and a drain electrode of the transistor 216 is electrically connected to the signal line 209 to which a data signal is supplied. The gate electrode of the transistor 102 is electrically connected to the scan line 207 to which a gate signal is supplied.

The transistor 216 has a function of controlling whether to write a data signal by being turned on or off.

The one of the source electrode and the drain electrode of the transistor 102 is electrically connected to a wiring 211 serving as an anode line, and the other of the source electrode and the drain electrode of the transistor 102 is electrically connected to one electrode of the light-emitting element 370. The gate electrode of the transistor 102 is electrically connected to the other of the source electrode and the drain electrode of the transistor 216 and the one electrode of the capacitor 105.

The transistor 102 has a function of controlling a current that flows to the light-emitting element 370 by being turned on or off.

One of a source electrode and a drain electrode of the transistor 217 is electrically connected to a wiring 210 to which a reference potential of data is supplied, and the other of the source electrode and the drain electrode of the transistor 217 is electrically connected to the one electrode of the light-emitting element 370 and the other electrode of the capacitor 105. A gate electrode of the transistor 217 is electrically connected to the scan line 207 to which a gate signal is supplied.

The transistor 217 has a function of adjusting a current that flows to the light-emitting element 370. For example, in the case where the internal resistance of the light-emitting element 370 is increased because of its deterioration, a current that flows to the wiring 210 electrically connected to the one of the source electrode and the drain electrode of the transistor 217 is monitored so that a current that flows to the light-emitting element 370 can be adjusted. A potential that is supplied to the wiring 210 can be, for example, 0 V.

The one of the pair of electrodes of the capacitor 105 is electrically connected to the other of the source electrode and the drain electrode of the transistor 102 and the gate electrode of the transistor 216. The other of the pair of electrodes of the capacitor 105 is electrically connected to the other of the source electrode and the drain electrode of the transistor 217 and the one electrode of the light-emitting element 370.

In the structure of the pixel 301 illustrated in FIG. 9C, the capacitor 105 serves as a storage capacitor that holds written data.

The one of the pair of electrodes of the light-emitting element 370 is electrically connected to the other of the source electrode and the drain electrode of the transistor 217, the other electrode of the capacitor 105, and the other of the source electrode and the drain electrode of the transistor 216. The other of the pair of electrodes of the light-emitting element 370 is electrically connected to a wiring 212 serving as a cathode.

The light-emitting element 370 may be an organic electroluminescent element (also referred to as an organic EL element) or the like, for example. Note that the light-emitting element 370 is not limited to an organic EL element; an inorganic EL element formed using an inorganic material may be used.

A high power supply potential VDD is supplied to one of the wirings 211 and 212, and a low power supply potential VSS is supplied to the other. In the configuration illustrated in FIG. 9C, the high power supply potential VDD is supplied to the wiring 211, and the low power supply potential VSS is supplied to the wiring 212.

In the display device including the pixel 301 in FIG. 9C, the pixels 301 are sequentially selected row by row by the scan line driver circuit 204, whereby the transistors 102 are turned on and a data signal is written.

When the transistors 102 are turned off, the pixels 301 to which the data has been written are brought into a holding state. Since the transistor 102 is electrically connected to the capacitor 105, the written data can be held for a long time. Furthermore, the transistor 216 controls the amount of current that flows between the source electrode and the drain electrode of the transistor 102. The light-emitting element 370 emits light with a luminance corresponding to the amount of flowing current. This operation is performed row by row sequentially, whereby an image can be displayed.

The structures, the methods, and the like described in this embodiment can be combined as appropriate with any of the structures, the methods, and the like described in the other embodiments.

Embodiment 4

In this embodiment, examples of electronic devices in each of which the display device of one embodiment of the present invention is included in a display portion will be described with reference to FIGS. 10A to 10H.

FIGS. 10A to 10H illustrate electronic devices. These electronic devices can include a housing 5000, a display portion 5001, a speaker 5003, an LED lamp 5004, operation keys 5005 (including a power switch or an operation switch), a connection terminal 5006, a sensor 5007 (a sensor having a function of measuring force, displacement, position, speed, acceleration, angular velocity, rotational frequency, distance, light, liquid, magnetism, temperature, chemical substance, sound, time, hardness, an electric field, a current, a voltage, electric power, radiation, a flow rate, humidity, gradient, oscillation, odor, or infrared ray), a microphone 5008, and the like.

FIG. 10A illustrates a mobile computer which can include a switch 5009, an infrared port 5010, and the like in addition to the above components. FIG. 10B illustrates a portable image reproducing device (e.g., a DVD reproducing device) which is provided with a memory medium and can include a second display portion 5002, a memory medium reading portion 5011, and the like in addition to the above components. FIG. 10C illustrates a goggle-type display which can include the second display portion 5002, a supporting portion 5012, an earphone 5013, and the like in addition to the above components. FIG. 10D illustrates a portable game machine which can include the memory medium reading portion 5011 and the like in addition to the above components. FIG. 10E illustrates a digital camera which has a television reception function and can include an antenna 5014, a shutter button 5015, an image receiving portion 5016, and the like in addition to the above components. FIG. 10F illustrates a portable game machine which can include the second display portion 5002, the memory medium reading portion 5011, and the like in addition to the above components. FIG. 10G illustrates a television receiver which can include a tuner, an image processing portion, and the like in addition to the above components. FIG. 10H illustrates a portable television receiver which can include a charger 5017 capable of transmitting and receiving signals, and the like in addition to the above components.

The electronic devices illustrated in FIGS. 10A to 10H can have a variety of functions. For example, a function of displaying a variety of data (a still image, a moving image, a text image, and the like) on a display portion, a touch panel function, a function of displaying a calendar, date, time, and the like, a function of controlling a process with a variety of software (programs), a wireless communication function, a function of being connected to a variety of computer networks with a wireless communication function, a function of transmitting and receiving a variety of data with a wireless communication function, a function of reading a program or data stored in a memory medium and displaying the program or data on a display portion, and the like can be given. Further, the electronic device including a plurality of display portions can have a function of displaying image data mainly on one display portion while displaying text data on another display portion, a function of displaying a three-dimensional image by displaying images where parallax is considered on a plurality of display portions, or the like. Furthermore, the electronic device including an image receiving portion can have a function of shooting a still image, a function of taking a moving image, a function of automatically or manually correcting a shot image, a function of storing a shot image in a memory medium (an external memory medium or a memory medium incorporated in the camera), a function of displaying a shot image on the display portion, or the like. Note that functions that can be provided for the electronic devices illustrated in FIGS. 10A to 10H are not limited thereto, and the electronic devices can have a variety of functions.

The electronic devices described in this embodiment each include the semiconductor device of one embodiment of the present invention in a display portion for displaying some sort of data.

The structures, the methods, and the like described in this embodiment can be combined as appropriate with any of the structures, the methods, and the like described in the other embodiments.

This application is based on Japanese Patent Application serial no. 2013-099915 filed with Japan Patent Office on May 10, 2013, the entire contents of which are hereby incorporated by reference. 

What is claimed is:
 1. A method for forming a display device comprising the steps of: forming a conductive layer over a substrate; forming a first electrode and a second electrode by patterning the conductive layer; forming a first insulating layer covering the first electrode and the second electrode; forming an oxide semiconductor layer over the first insulating layer; forming a first oxide semiconductor layer overlapping with the first electrode and a second oxide semiconductor layer overlapping with the electrode by patterning the oxide semiconductor layer; forming a source electrode and a drain electrode over and in contact with the first oxide semiconductor layer; forming an oxide insulating layer over and in contact with the first oxide semiconductor layer, the source electrode, and the drain electrode; forming a first opening in the oxide insulating layer, the first opening overlaps with the second oxide semiconductor layer and the second electrode; and forming a pixel electrode from the second oxide semiconductor layer, wherein the step of forming the pixel electrode comprises a step of: forming a second insulating layer containing hydrogen over and in contact with the second oxide semiconductor layer in the first opening, wherein the first oxide semiconductor layer serves as a channel formation region, wherein the second electrode, a portion of the first insulating layer, and a portion of the pixel electrode form a capacitor, and wherein the oxide semiconductor layer contains indium, zinc, and a metal element.
 2. The method for forming the display device according to claim 1, wherein the metal element is any one of Al, Ti, Ga, Y, Zr, La, Ce, Nd, and Hf.
 3. The method for forming the display device, according to claim 1, wherein the second insulating layer is a silicon nitride layer with a hydrogen concentration of 1×10²² atoms/cm³ or higher.
 4. The method for forming the display device, according to claim 1, further comprising a step of: performing first heat treatment on the first oxide semiconductor layer and the second oxide semiconductor layer; and performing second heat treatment after forming the oxide insulating layer.
 5. The method for forming the display device according to claim 1, wherein an amount of oxygen vacancies of the first oxide semiconductor layer is lower than an amount of oxygen vacancies of the pixel electrode.
 6. The method for forming the display device according to claim 1, wherein a resistivity of the pixel electrode is lower than a resistivity of the first oxide semiconductor layer.
 7. A method for forming a display device comprising the steps of: forming a conductive layer over a substrate; forming a first electrode and a second electrode by patterning the conductive layer; forming a first insulating layer covering the first electrode and the second electrode; forming an oxide semiconductor layer over the first insulating layer; forming a first oxide semiconductor layer overlapping with the first electrode and a second oxide semiconductor layer overlapping with the electrode by patterning the oxide semiconductor layer; forming a source electrode and a drain electrode over and in contact with the first oxide semiconductor layer; forming an oxide insulating layer over and in contact with the first oxide semiconductor layer, the source electrode, and the drain electrode; forming a first opening in the oxide insulating layer, the first opening overlaps with the second oxide semiconductor layer and the second electrode; forming a pixel electrode from the second oxide semiconductor layer; wherein the step of forming the pixel electrode comprises a step of: forming a second insulating layer containing hydrogen over and in contact with the second oxide semiconductor layer in the first opening, and forming a liquid crystal layer over the pixel electrode, wherein the first oxide semiconductor layer serves as a channel formation region, wherein the second electrode, a portion of the first insulating layer, and a portion of the pixel electrode form a capacitor, and wherein the oxide semiconductor layer contains indium, zinc, and a metal element.
 8. The method for forming the display device according to claim 7, wherein the metal element is any one of Al, Ti, Ga, Y, Zr, La, Ce, Nd, and Hf.
 9. The method for forming the display device, according to claim 7, wherein the second insulating layer is a silicon nitride layer with a hydrogen concentration of 1×10²² atoms/cm³ or higher.
 10. The method for forming the display device, according to claim 7, further comprising a step of: performing first heat treatment on the first oxide semiconductor layer and the second oxide semiconductor layer; and performing second heat treatment after forming the oxide insulating layer.
 11. The method for forming the display device according to claim 7, wherein an amount of oxygen vacancies of the first oxide semiconductor layer is lower than an amount of oxygen vacancies of the pixel electrode.
 12. The method for forming the display device according to claim 7, wherein a resistivity of the pixel electrode is lower than a resistivity of the first oxide semiconductor layer. 